1. Field of the Invention
The present invention relates to an accumulator of an automatic transmission mounted on an automobile, particularly to a structure to hold a piston and a coil spring of the accumulator.
2. Description of the Prior Art
Generally an automatic transmission has a planetary gear mechanism, and certain elements in the mechanism are engaged or restrained by clutches or brakes (frictional engaging element) so that desired speed is obtained. These brakes and clutches are controlled by a hydraulic servo wherein oil pressure is applied or drained by switching of a shift valve. The hydraulic servo has an accumulator beside it, and according to hydraulic characteristic of the accumulator, characteristics of the hydraulic servo or those of the clutch and brake are determined.
Certain accumulator, such as for second brake functioning at second speed, for an over drive brake functioning at over-drive condition, for an over-drive direct clutch functioning changing from first to third speed, or for a direct clutch functioning at reverse condition and third speed, is preferable to obtain certain characteristic of accumulator by installing a coned disc spring or a coil spring which natural length is shorter than that of the piston stroke in an accumulator chamber so that such coil spring functions only at a start of the piston stroke. In this case, if a coned disc spring is used, it is difficult to set desired hydraulic characteristic because a coned disc spring has smaller degree of freedom on load capacity and coefficient of elasticity. On the other hand, if a coil spring is used and fixed on the housing, it tends to lean. And if the piston contacts the coil spring while the coil spring leans, the piston can not come to the bottom end of stroke, so that large shift shock may develop.
Then the applicant of the present invention proposed an accumulator which fixes its coil spring to the piston by caulking, bolting or clamping margin of the coil spring to prevent leaning of the coil spring as shown in the Japanese patent publication No. 61-48602.
Recently an automatic transmission has been mounted on small horse-powered cars, and an accumulator has been required to be small because of the tendency of so-called front-engine and front-drive cars.
In the above case, fixing the coil spring by caulking or bolting is quite difficult because the inner diameter of the coil spring becomes small, and keeping the coil spring vertical is also quite difficult. On the other hand, pressing by the clamping margin is easy to come off. If the clamping margin is made larger to prevent such coming off it is possible to press the coil spring.
The above clamping, bolting and pressing need larger area for fixing, so a large hole is necessary for the fixing area not to interfere with the housing. Accordingly in the case of an accumulator whose piston is installed upward, a concave area tends to catch air. The air occupies comparatively large area in the hole, so the characteristic of the accumulator may vary from the desired value, and may cause shift shock.